1. Field of the Invention
The present invention relates to a driving voltage controller for controlling a driving voltage to drive a load such as a liquid crystal display panel by using an alternating current.
2. Description of the Related Art
FIG. 18 shows an example of a related art driving voltage controller for controlling a load such as a liquid crystal display panel. Thus, on a related art voltage controller are provided a High output operational amplifier OPVcomH and a Low output controller operational amplifier OPVcomL at the High/Low output of a power source generating a counter voltage Vref, as shown in FIG. 18. An output switch 10 of MOS transistors drives these operational amplifiers alternatively in order to supply an alternating voltage to a liquid crystal display panel.
An operational amplifier which is not connected to the liquid crystal display panel (hereinafter referred to as a “deselected operational amplifier”) has different frequency compensating conditions between the panel-load conditions and the non-load conditions. Thus, an operational amplifier stable under the panel-load conditions could be unstable under the non-load conditions and oscillate in an unstable operation. To cope with this problem, the output of each operational amplifier is connected to a capacitor (stabilizing capacitance) C for stabilization purposes. In order to regulate the bias current source thereby reducing power consumption, a register (not shown) which can be set through a command sent from a CPU is internally provided. In the sleep or standby state, it is possible to change the setting of the register to shut down the operation of the operational amplifier, thus reducing the power consumption.
In the aforementioned related art driving voltage controller, the capacitor C is provided, which adds to the number of mounted components and the corresponding cost, as well as limits the mounting area of components. Typically, a capacitor becomes larger as its capacitance increases. Thus, a driving voltage controller has been in need which can supply an alternating voltage in a stable fashion free from oscillation of a deselected operational amplifier without the capacitor C.
The output switch 10 for switching between operational amplifiers preferably has no resistance. Increasing the W/L size ratio of the switch expands the capacity of the diffused section of the switch, which capacity acts as a load on the operational amplifier thus increasing the driving power and preventing a low-power design. The bias current flowing in an operational amplifier, determined by a value set to the register by the CPU considering the power supply capacity and power consumption, can be dynamically changed depending on the operating state of the operational amplifier.